1. Field of the Invention
The present invention relates to methods of detecting strain in birefringent materials.
2. Background of the Related Art
Strain is a geometrical expression of deformation caused by the action of stress on a physical body. Strain therefore expresses itself as a change in size and/or shape, and is typically measured as a percentage elongation.
When transparent, isotropic materials are squeezed, bent or stretched to become anisotropic, birefringence can result. Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain types of material, such as calcite crystals, depending on the polarization of the light. This effect can occur only if the structure of the material is anisotropic. Many plastics are birefringent, because their molecules are ‘frozen’ in a stretched conformation when the plastic is molded or extruded. Alternatively, many plastics such as cellophane exhibit birefringence when strained.
A pressurized balloon formed of polyethylene film is a good example of a birefringent material that may become strained. One existing method for monitoring balloon strain in flight uses permanent marks on the balloon with a known distance between them at zero strain to determine stretch via a video inspection feed. Another method uses distributed piezoelectric strain gauges on the surface of the balloon and relies on the electrical signal produced in the gauges as the film stretches. A third approach involves the monitoring of differential Global Positioning System (GPS) receivers arrayed around the balloon surface to determine stretch in the film. However, each of these approaches has shortcomings which limit its functionality and practicality for this application.
The use of photogrammetry is currently the most viable method of measuring strain in balloons. As the balloon expands, the distance between marks on the balloon increases and the length increase can be measured directly. However, the requirement that multiple cameras be used for triangulation of point positions complicates this measurement approach significantly. Additionally, measurements can only be taken from discrete points on the balloon where marks had been made. Even when the film could be directly measured, this measurement method is limited to an accuracy of approximately 0.5%, with the values obtained representing an average over the entire range. This resolution limit can lead to small defects being missed.
Distributed strain gauges are inherently noisy and do not provide data from much of the balloon's surface. High noise levels in the signals make it difficult to detect the small changes that can be precursors of a larger failure. Averaging the data over time can improve sensitivity but it can delay detection of an impending failure. In addition, because strain gauges can only report changes in the local area around the strain gauge, localized failures between strain gauges can go undetected. Increasing the density of the strain gauges can improve the coverage, but more gauges and their connections also increase the weight of the system. This increase reduces the load capacity of the balloon, which means less instrumentation can be carried.
Using differential GPS (DGPS) is logistically complex and requires careful mounting of GPS receivers over the entire surface of the balloon. This type of system is less sensitive, and cannot detect point failures until they are large enough to deform the entire balloon shell. Current, state-of-the-art DGPS receivers have a relative accuracy measured in centimeters. Therefore, this approach would only be practical for monitoring large-scale balloon deflections. Like strain gauges, increasing receiver density can improve the coverage. However, such an increase in receiver number would increase the weight of the system and reduce the load capacity of the balloon. An effective system for measuring the strain on the balloon would preferably monitor the entire surface of the balloon with no need for an extensive network covering the surface of the balloon. Ideally, such a system would be able to detect impending failure of all sizes and from a variety of causes while adding a minimum of weight.
All the methods described above have fundamental limitations that severely limit the precision and accuracy capabilities of a balloon strain monitor. Therefore, there is a need for an improved method and system for measuring strain in birefringent materials, such as a plastic film forming a balloon. It would be desirable if the method and system were adaptable to measuring strain in birefringent materials in a variety of applications, including plastic film production processes and quality control. Ideally, such a method and system would provide spatially continuous monitoring of the birefringent material. Furthermore, it would be desirable if the method and system provided greater accuracy than any of the other existing technologies described above.